1. Field of the Invention
The present invention relates to an image forming apparatus such as a copier and a program of the image forming apparatus, and particularly to an image forming apparatus having a function to write an image of multiple scanning lines in a single scanning process onto a recording medium such as a photoreceptor by utilizing laser beams from multiple light sources, and a control program thereof.
2. Description of the Related Art
An image forming apparatus is known which forms one line image in a main scanning direction based on image data, and forms one page of image by repeating the image formation of the main scanning direction line by line in a sub-scanning direction.
As an example, an electrophotographic image forming apparatus scans a laser beam modulated based on image data in the main scanning direction onto an image carrier, and in parallel to the main scanning, forms an image by the use of the laser beam on the image carrier (photoreceptor drum) rotating in the sub-scanning direction. In this case, by the reference of a clock signal (pixel clock), which is also called a dot clock, the laser beam is modulated based on the image data.
Further an image forming apparatus is known which is provided with two or more light sources such as laser diodes (LD), uses laser beams from the multiple light sources, and by repeating the image formation of multiple lines in the main scanning direction based on the image data in the sub-scanning direction, performs image formation of a single page. The multi-beam type image forming apparatus is described in such as Unexamined Japanese Patent Application Publication No. 63-124664 (abbreviated to JPA63-124664).
Further, regarding the multi-beam type image forming apparatus, various types are described in such as JPA63-124664, JPA2002-166592, and JPA2005-288773.
Although it is possible to form an image on a transfer sheet other than normal paper, such as a sheet of thick paper which needs to pass through a fixing section at a slower speed. In such case, it is necessary to lower the transporting speed of the transfer sheet.
In such low speed image formation, if the rotating speed of a polygon mirror is changed, since the load onto a polygon motor is increased, it is not preferable for using the image forming apparatus stably for a long period.
In this regard, there is a method to change the image formation speed in the multi-beam image formation, by decreasing the number of uses beams, namely the number of used light sources, without changing the rotating speed of the polygon mirror.
However, in this case of decreasing the number of used light sources, the difference of light emitting period between the used light source and the unused light source is generated, and a problem results in that the state of light source deterioration differs by each of the light sources.
In JPA63-124664, although technologies regarding the multi-beam type image forming apparatus are generally described, the difference of the deterioration state between the multiple light sources is not taken into account.
In JPA2002-166592, technology regarding beam switching is disclosed, however this causes a new problem in that the program capacity (memory capacity) must be increased due to the complicated control program.
Further, in JPA2005-288773, a technology is disclosed which equalizes the deterioration by switching the light sources to be used for every page when there is a mode where any of the light sources is not used. In this case, it is necessary to control the switching of light sources page by page, and a new problem may be generated in that image quality changes page by page due to the switching of the light sources. This image quality change may be a serious problem since this leads to a color shift in case of color image formation.
The present invention is accomplished to solve the abovementioned problems, and to provide an image forming apparatus and control program of the image forming apparatus which prevents the deterioration of image quality, by equalizing the deterioration state of each light source as much as possible when multiple exposure laser beams are used, and thereby the light sources can be stably used.